1. Field of the Invention
The present invention relates to a magneto-optical medium which records information in an ultrahigh density and reproduces the information by utilizing displacement of a magnetic domain wall. The present invention relates also to a process for producing the magneto-optical medium.
2. Related Background Art
In recent years, magneto-optical disks are attracting attention as a rewritable high-density recording medium. However, the magneto-optical medium is required to be capable of memorizing information in higher density for a larger memory capacity. The linear memory density of the optical disk depends on the laser wavelength .lambda. and the numerical aperture NA of the objective lens of the reproducing optical system. The detectable spatial frequency in signal reproduction is limited to about NA/.lambda..
For a higher memory density of a conventional optical disk, the laser wavelength .lambda. should be made shorter, and the numerical aperture NA of the objective lens should be larger. However, the improvements in the laser wavelength and of the numerical aperture of the objective lens are naturally limited. Thus, several techniques for increasing the memory density by improving the constitution of the recording medium or the read-out method are disclosed.
For example, the inventor of the present invention disclosed, in Japanese Patent Application Laid-Open No. 6-290496, a magneto-optical medium capable of reproducing signals of a frequency of less than an optical diffraction limit at a high speed, a reproduction system and a reproduction apparatus therefor. In the reproduction layer of the disclosed magneto-optical medium, the magnetic domain walls are displaced toward a portion of lower domain wall energy by causing a temperature distribution by means of a heating means such as a light beam, owing to formation of distribution of the domain wall energy. Accordingly, the amplitude of the reproduced signals is constant and maximal independently of the interval of recorded domain walls (i.e., recording bit length). Accordingly, by this feature, the extent of reduction in reproduction output, which is accompanied inevitably with increase of the linear memory density, is decreased, and further higher memory density is achievable.
The inventors of the present invention disclosed, in Japanese Patent Application No. 9-289719, now Japanese Patent Publication 11-191245, control of the surface roughness of the magnetic layer (i.e., substrate surface) to be small to facilitate the displacement of the magnetic domain in the displacement layer in the constitution of the medium disclosed in the aforementioned Japanese Patent Application Laid-Open No. 6-290496. According to the disclosure, the surface roughness of the substrate is directly reflected to the surface roughness of the displacement layer. The smaller the surface roughness, the more the mobility of domain walls is increased because the surface roughness retards the displacement of the domain walls.
However, although the above disclosed technique is effective for facilitating the domain wall displacement, the surface roughness of the memory layer also becomes less, which lower undesirably both the coercivity of the memory layer and the preservability of the domains which is the inherent role of the memory layer. The drop of the magnetic domain preservability is a serious problem, particularly in recording in finer domains.